Relations between organic matter and bacterial proteolytic activity in sediment surface layers of a eutrophic lake (Lake Aydat, Puy de Dome, France)

Seasonal changes in Leucine-aminopeptidase activity in surface layers of an oxic sediment rich in organic matter were studied from July to October 1997 in a eutrophic lake. The mean bacterial density varied between, 119 +/- 20 109 cells/g DS with a biomass of 14 +/- 3 mgC/g DS. The average Leucine-am inopeptidase activity was 487 +/- 177 nmole h(-1) g(-1) DS over the entire sampling period. It consisted almost entirely of serine and cysteine protea ses which had a pH optimum between 8 and 9 and an optimal temperature betwe en 20 degrees C and 40 degrees C. In order to study which factors determine this activity, the inputs of organic matter from the trophogenic zone and the concentrations of dissolved proteins and carbohydrates in the sediment were measured. The Leucine-aminopeptidase activity seemed to be repressed b y organic matter (OM) flow (r = -0.575, P<0.05) and by carbohydrates (r = - 0.684, P<0.05), and induced by dissolved proteins (r = 0.633, P<0.01) throu ghout the study period.